Brake-Specific Fuel Consumption - Enginology

Whether you're an engine builder or engaged in testing them on an engine dynamometer, a clear and working understanding of brake-specific fuel consumption (BSFC) can be of value. The broader category includes parts designers and those interested in evaluating power level changes involving parts or modifications. In one way or another, any changes in power (positive or negative) can be linked to combustion efficiency. And, simply stated, BSFC is keyed to this as well. Despite previous discussions about the subject, we'll expand on it a bit in this presentation.

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Now, although "Enginology" is not intended to include an array of mathematical calculations in support of the information provided, it's worth noting how to compute BSFC because that will help in understanding the importance of its numerical relationships. In the English system of units, the computation involves fuel flow in pounds per hour (pph) and "observed" horsepower (uncorrected for barometric pressure and inlet air temperature). Arithmetically, if we divide fuel flow by observed horsepower, the units of measure will be pounds/horsepower-hour. That's the academic perspective.

As a practical matter, BSFC is a measure of how efficiently a given amount of fuel is being converted into a specific amount of horsepower. More broadly stated, it could also be considered a measure of combustion efficiency, and that's key to our discussion, but first we need to include some thoughts about a related subject.

Regardless of the type of fuel being used, it has a specific energy content for a given volume. That means if we were to burn all the fuel and capture all the heat delivered during any particular combustion cycle, we would have extracted the maximum amount of potential horsepower. Unfortunately, however, the internal combustion engine is not an efficient one. And while you can expect certain percentages of energy content will be lost to the exhaust and cooling systems, they can run in the range of a 20-25 percent loss to each system, in the best of cases.

It's not uncommon for these percentages to be higher. So the objective in building, modifying, or tuning a racing engine is to minimize these unavoidable losses. For example, thermal coatings intended to reduce heat losses to the cooling system are attempts to increase the amount of energy focused in the combustion space. The same applies to coating major exhaust system components, like headers. Makes sense.

Stated another way, we're talking about improving an engine's "thermal efficiency" by minimizing heat losses, particularly to the cooling and exhaust systems. As this is accomplished, power stands to increase, and we need a way to evaluate what's going on in the combustion space. This brings us full circle and back to using BSFC as the yardstick. Short of conducting in-cylinder pressure analysis tests that are comparatively more expensive and complex than considering BSFC data, how do we do this?

First, let's consider a practical example. Suppose we're evaluating a gasoline-fueled racing powerplant on an engine dynamometer. At wide-open throttle, full load, and constant rpm (using race gas), the "chemically correct" baseline BSFC was some time ago considered to be 0.500 pounds of fuel flow/horsepower-hour.

As engine builders and modifiers refined ways to improve both thermal and combustion efficiency by methods that included combustion chamber shapes, piston crown designs, exhaust system efficiency, and related areas, the original "standard" for gasoline decreased to somewhere only slightly north of 0.400. This meant that improved combustion was allowing the same amount of fuel to produce an increase in power—e.g., combustion efficiency improved. As a result, BSFC was reduced.

Right about here, it might be worthwhile to note something. Let's say we have two engines of approximately equal power. That is to say their respective torque and horsepower curves are quite similar. It happens with some frequency. However, one exhibits a BSFC curve with values lower than the other engine. It turns out that the one with the lower BSFC curve will accelerate more quickly than the other, once on the track—all else being equal. It will also be more responsive to changes affecting combustion efficiency as well and will very likely require less total spark timing to perform at its best. On-track fuel economy will also be superior. But we digress.

Overall, there are some truths and there seems to have been a bit if misunderstanding about what BSFC data are actually indicating. For example, if an engine is experiencing an inordinate (maybe even some) combustion contamination from residue (exhaust gas) left in the combustion space during succeeding combustion events, BSFC data generally increase numerically.

In such cases, exhaust gas temperatures (EGT) tends to trend downward. This is often accompanied by the necessity for additional fuel and more spark timing, in an attempt to resolve the problem. Conditions like this simply mean the contamination problem requires correcting.

One approach I observed a number of years ago and found to be helpful is the following method of using BSFC as a tool. Since an engine tends to be the most combustion efficient at or near peak torque, it will save wear and tear on an expensive engine (or even one not as expensive) to perform initial spark and fuel calibration tests at peak torque. There is a reason why BSFC tends to be numerically the lowest at this point, but we'll get to that in a moment.

By using this method, it's possible to minimize the engine's test burden but make possible the pinpointing of the best (lowest) BSFC by adjusting spark and fuel…separately. First, find the best spark setting. Then you can adjust fuel flow until the lowest BSFC is determined without an attending loss in power. Simple enough.

Ideally, you'd like to create a "flat" BSFC curve, but that's not entirely possible. What you can do is work toward establishing that condition, using the value of BSFC at peak torque as your target. However, a couple of conditions play into any attempt at "flattening" a BSFC curve. Among them is the fact that at engine speeds below peak torque, there's an increased amount of time for heat losses to the cooling system and related parts and passages. And especially if the engine is using a carburetor, air/fuel charge quality tends suffer from less efficient atomization (mixing) at these engine speeds than higher rpm.

Beyond peak torque, there is the issue of mechanical separation of air and fuel as signified by a corresponding rise in BSFC values. In other words, combustion efficiency tends to deteriorate, accordingly. Plus, in the higher rpm ranges, while there is an increase in the amount of otherwise useable heat from combustion, high rpm shortens the time available to deliver that heat, resulting in a loss in power.

So there you have it. Given contemporary technology and equipment availble to qualify and quantify the combustion process, using BSFC analysis has become somewhat of a "poor man's" method of tuning or modifying a racing engine, but it works and is far more economical than some of the other methods. Just keep in mind that it can also be useful when evaluating parts of modifications that relate the combustion process. Although that was previously mentioned, the technique can be a valuable approach to determining the best package of engine components and level of tune before you head to the track.